1. Human Origins and Evolution Chapter 24
We can approach the question of our place in the tree of life in three different ways:
comparative anatomy
molecular analysis
the fossil record.

2. Primate Family Tree
24.1 Anatomical, molecular, and fossil evidence shows
that the human lineage branches off the great apes tree.
Tamarins, titis, capuchins
Baboons, mandrills, macques
Premonkeys
Anatomical features show that primates are a monophyletic group of 400 species….that include prosimians, monkeys, and apes. All primates share a number of general features that distinguish them from other mammals (opposable thumbs, eyes on the front of their face, nails instead of claws). Monkeys underwent independent bouts of evolutionary change in the Americas and in Africa and Eurasia, so the family tree is split between New and Old World monkeys.
Primates are a monophyletic group of 400 species. All primates:
Opposable thumbs, hands and feet for grasping, eyes on front of face, nails instead of claws,

3. Ape Family Tree
One line of Old World monkeys gave rise to apes: which lack a tail and show more sophisticated behaviors than other monkeys. The apes are split into two groups: the lesser and the great apes. Lesser apes include fourteen species of gibbon, which are all found in Southeast Asia. The great apes include the orangutan, gorilla, chimpanzee, and humans.
Sequence analysis differences show: humans split from chimps about 5 –7 mya
Apes split off from Old World Monkeys and lost their tail. 2 groups of apes:
Lesser apes: 14 species of gibbons, all in SE Asia
Great apes: orangutan, gorilla, chimps, and humans
Humans split from chimps 5-7 mya

4. Molecular Analysis of Human and Chimpanzee DNA
Constructing a phylogenetic tree for humans and primates based on comparison of anatomical features was unsuccessful because gorillas and chimpanzees are too physically similar. Only molecular analysis of human and primate DNA could reveal that our lineage split from the chimpanzee lineage about 5−7 million years ago.
Prior to the development of DNA sequencing technology, Mary-Claire King and Allan Wilson from the University of California at Berkeley used DNA-DNA hybridization methods to determine how much genetic difference there is between humans and chimpanzees.
When two complementary strands of DNA are heated to 95°C, the hydrogen bonds holding the two strands together break down and the double helix denatures, or separates. The more mismatched the two strands of DNA, the lower the temperature required to split them apart because there are fewer hydrogen bonds holding the strands together. King et al. found that the human-chimpanzee hybridized DNA separated at a temperature of about 1°C less than the temperature at which human-human DNA separated.
This difference was calibrated on the basis of studies of other species whose genetic difference was known. 1°C difference in denaturation temperatures equates to about 1% in DNA sequence.
We can all agree that the difference genetically may be 1%, but the difference in behavior and appearance is quite different. This is attributed to the differences in gene regulation between the two species.
10C diff in denaturation temperatures = 1% diff in DNA sequence
Huge diff in appearance and behavior
Due to gene regulation between 2 species

5. QUICK CHECK
Did humans evolve from chimpanzees? Explain.

6. ANSWER No. Modern humans and modern chimps share a common ancestor.
Changes have occurred along both lineages: from common ancestor to modern humans, and from the common ancestor to the modern chimps.

7. Earliest Hominin (Sahelanthropus tchadensis)
Hominin: all of the different species in the lineage leading to humans
Earliest hominin:
S.tchadensis about 7 million years old
found in Chad in 2002
Chimp-sized brain with hominin-type brow ridges
For a molecular analysis, we need living individuals, but since these individuals have long disappeared, we must get a picture of human evolution from fossils.
The members of all the different species in the lineage leading to humans are called hominins. The earliest known member of the hominin lineage is Sahelanthropus tchadensis. This specimen was discovered in 2002 in Chad. It combines both modern human and ancestral features. It has been dated to be about 7 million years old and has a chimpanzee-sized brain but hominin-type brow ridges.

8. Lucy (Australopithecus afarensis)
Found in Ethiopia in 1974
Most complete hominin fossil
3.2 million years old
Fully bipedal
In 1974, researchers found another early hominin fossil in Ethiopia, dating from around 3.2 million years ago, and called her Lucy. Lucy represents the first fully bipedal hominin to habitually walk upright (bipedal). She was less than 4 feet tall and had a considerably smaller brain than modern humans.

9. Hominin Lineages
At times, there were as many as 3 species living in Africa at the same time.
1st hominins to venture out of Africa:
Homo ergaster (aka Homo erectus)
2 mya
Homo neanderthalensis:
Descendent of Homo ergaster
Closely related to Homo sapiens
Thicker bodied than us with flatter head, and brains same size
1st appeared in fossil record 600,000 ya and disapperared 30,000 ya
The hominin lineage produced many different species in Africa. At times, there were as many as three species living at the same time.
All hominins have a common ancestor, but not all of them lead to modern humans, which are called Homo sapiens. Instead, these branches ultimately went extinct.
The fossil record indicates that the first hominin lineage to venture out of Africa was Homo ergaster (also called Homo erectus), and did so about 2 million years ago. They were the first hominin in the Homo genus. Fossils have been found throughout Eurasia but not in Australia or the Americas.
Homo neanderthalensis was a descendent of H. ergaster and was closely related to Homo sapiens. Its fossils were found in Europe and the Middle East. Neanderthals were thicker boned than us and had flatter heads with brains about the same size as us. Neanderthals first appeared in the fossil record around 600,000 years ago and disappeared around 30,000 years ago.
Homo floresiensis, also popularly known as the Hobbit, was another descendent of H. egraster that became extinct 12,000 years ago,. This species was limited to the Indonesian islands of Flores. Adults were just over 3 feet tall.

10. Cranium Size Increase Body size increased
Tripling of brain and cranium
(occurred after bipedalism)
A number of trends can be seen when looking over the entire record. Body size increased, as did the size of the cranium (which is inferred to also include the size of the brain).
Tripling of brain size…..occurred after bipedalism
From 400 cm3 to 1200 cm3

11. Origins
24.2 Phylogenetic analysis of mtDNA and the Y chromosome shows that our species arose in Africa.
Multiregional Hypothesis:
modern H. sapiens derived from multiple H. ergaster populations that spread around the world 2 mya
Implies that modern traits evolved convergently in multiple populations
Out-of-Africa Hypothesis:
modern H. sapiens arose from H. heidelbergensis (H. ergaster descendents) in Africa about 200,000 years ago
Implies one evolutionary origin for modern traits
There have been two hypothesis of modern human origins.
The multiregional hypothesis suggested that modern humans, Homo sapiens, derive from the Homo ergaster populations that spread
around the world starting about 2 million years ago, implying that different Homo ergaster populations throughout the Old World evolved in parallel, with some limited gene flow among them, to each produce modern H. sapiens populations. In short, modern human traits evolved convergently in multiple populations.
The out-of-Africa hypothesis, developed in 1987 from another study from Allan Wilson’s laboratory, instead suggested that modern humans arose much more recently from Homo ergaster descendants (sometimes called Homo heidelbergensis) in Africa, about 200,000 years ago. This hypothesis implies that there was one evolutionary origin for modern human traits.

12. Mitochondrial DNA (mtDNA)
Small circle of DNA (about 17,000 base prs.long)
Found in every mitochondrion
Each cell has many mitochondrions
Therefore, mtDNA much more abundant than nuclear DNA
Also easier to extract
All your mtDNA comes from mother’s egg, since sperm do not contribute mitochondria to zygote

13. Variation in restriction sites in human populations
Using 12 diff restriction enzymes to cut mtDNA from living people
If restriction sequence was present, the enzyme would cut
If sequence was mutated, the DNA would not be cut
Rebecca Cann, a student in Allan Wilson’s laboratory, chose to analyze DNA sequences to reconstruct the human family tree. Cann used mt DNA from living people around the world to determine which theory was correct. She inferred differences among sequences by digesting the mtDNA with different restriction enzymes. If the restriction sequence was present, the DNA would be cut, and if the sequence was mutated so that it was no longer recognized by the restriction enzyme, it would remain uncut. The resulting fragments were separated by gel electrophoresis. By using 12 different enzymes, she was able to assay a reasonable proportion of all the mtDNA sequence variation present in the sample.

14. Phylogenetic Tree Based on Restriction Site Variation
Study showed that modern
humans arose relatively recently in Africa, and their most recent ancestor lived 200,000 ya.
The multiregional hypothesis is rejected.
All non-Africans derive from within the African family tree.
Variation in restriction sites can be used to construct phylogenetic trees in the same was as anatomical features. By mapping the changes of shared derived characters, Cann could construct the phylogenetic relationships among the 12 samples. For example, she was able to deduce that, because there was a cut at site 5 for the Japanese and Native American sequences, the two groups had a common ancestor with a mutation that created the new cut at site 5.

15. Support for the out-of-Africa theory
Cann’s tree shows that modern humans arose from Africa about 200,000 years ago. Her findings resulted in the multiregional hypothesis being rejected and gave rise to the out-of-Africa theory.
Modern humans arose from Africa 200,000 ya: Out-of-Africa Hypothesis

16. Neanderthals Neanderthals disappeared from Europe
about 30,000 ya when Homo sapiens (called Cro-Magnons) 1st appeared in Europe
Did Cro-Magnons interbreed with H. neanderthals?
If so, they would be our descendents.
Neanderthals disappeared from Europe about 30,000 years ago when early H. sapiens (called Cro-Magnons) first appeared in Europe.
So what happened to H. neanderthals? Did they interbreed with Cro-Magnons? If so, they would be our descendents.

17. The Evidence for and Against Interbreeding
In sequencing entire Neanderthal genome:
Our ancestors DID interbreed with Neanderthals
1-4% of the genome of every non-African is Neanderthal-derived
2 possibilities for Discrepancy:
Neanderthal mtDNA was lost through
genetic drift
2. Female Neaderthals did not breed with
our ancestors, but the males did
In 1997, Svante Paabo’s lab in Germany extracted mt DNA from 30,000-year-old Neanderthal material and found that their sequences were strikingly different from those of modern humans, suggesting that there was no interbreeding with our ancestors.
However, this conclusion was reversed when others were able to sequence the entire Neanderthal genome, not just its mt DNA, and compare it to that of our ancestors. It was found that our ancestors did interbreed with Neanderthals and 1−4% of the the genome of every non-African is Neanderthal-derived.
How can we reconcile these two contradictory results? Two possibilities:
Neanderthal mt DNA was lost through genetic drift.
Female Neanderthals did not breed with our ancestors but males did: Since mtDNA is from the mother, there would be no Neanderthal mtDNA in modern humans if female Neanderthals did not interbreed.

18. Becoming Bipedal
24.3 During the 5-7 million years since the most recent common ancestor of humans and chimpanzees, our lineage acquired a number of distinctive features.
Full bipedalism: 3.5 mya
Involved anatomical changes in:
Skull
Spine
Legs
feet
Changes in anatomy and behavior occurred in the 5−7 million years since our lineage split from the lineage that yielded the chimpanzees.
One of these features was the shift from walking on four legs to walking on two. As we have already observed, Ardi was partially bipedal and Lucy was fully bipedal. We can refine our estimate of when full bipedalism arose from the evidence of a trace fossil. A set of fossil footprints dating from 3.5 million years ago in Tanzania reveal a truly upright posture.
To become bipedal, a number of anatomical characteristics changed in the skull, spine, legs, and feet.

19. Neoteny DNA of humans and chimps are 99% similar
Attributed the differences between humans and chimps to gene regulation
Humans are neotenous; exhibiting many traits as adults that chimps exhibit as juveniles
Hairless face
Small teeth
Flattened face
Hairless body
Large heads
Position of foramen magnum
King and Wilson’s 1975 discovery that the DNA of humans and chimpanzees were 99% similar have now also been confirmed by DNA sequence comparison of their genomes. In King and Wilson’s original paper, they attributed the differences between the two species to gene regulation.
One of the gene-regulatory pathways that changed may be responsible for human neoteny. Neoteny is the long-term evolutionary process in which the timing of development is altered so that a sexually mature organism still retains the physical characteristics of the juvenile form. Adult humans share many characteristics with juvenile chimpanzees.
Neoteny: juvenilization: retention by adults of traits previously seen only in the young

20. Brain Size Humans have large brains for their body size.
Possibilities:
Tool use
Social living….coordination
Language
In mammals, brain size is typically correlated with body size. Humans are relatively large-bodied mammals, but our brains are large even for our body size. Because a large brain is metabolically expensive to produce and maintain, we can conclude that natural selection must have acted in favor of large brains. Why?
Tripling of brain size
Tool use: Manual dexterity requires a complex nervous system.
Social living: coordination
Language

21. FOXP2
The differences between humans and chimps lies in the 1% DNA differences.
FOXP2 gene:
Encodes transcription factors that play an
important role in development
Mutation in this gene:
Difficulty with speech and language
The differences between humans and chimpanzees lies in the approximately 1% of DNA that differs between the two organisms. One gene of interest is the FOXP2 gene, which encodes transcription factors that play important roles in development. It has been shown that individuals with mutations in this gene have difficulty with speech and language, and other animals with this gene knocked out also have communication impairments.
Studies of the gene reveal a pattern of extreme conservation. The sequences in mice and chimpanzees only differ by one amino acid. And the sequence in humans contains two additional amino acids that are not present in the chimpanzee. Interestingly, Neanderthals also had the modern human version of this gene.

22. QUICK CHECK
The FOXP2 gene is sometimes called the “language gene”. Why is this name inaccurate?

23. ANSWER
1. The gene is expressed in many tissues, so its effects are not limited to language and speech.
2. Many genes are required for language, not just FOXP2.
3. Also, its association with vocal communication in multiple species is striking.

24. Human Migratory Routes
24.4 Human history has had an important impact on patterns
of genetic variation in our species.
Homo sapiens arose in Africa as determined by Cann’s mtDNA experiments. About 60,000 years ago, populations started to venture out through the Horn of Africa into the Middle East. The first phase of colonization took our ancestors through Asia and into Australia about 50,000 years ago. Not until about 15,000 years ago did the first modern humans cross from Siberia to North America to populate the New World.
Despite its closeness to the African mainland, the first humans arrived in Madagascar only about 2000 years ago, and the colonists came from Southeast Asia, not Africa.
Hawaii was colonized about 1500 years ago and New Zealand only 1000 years ago.
Because the spread of modern humans was so recent, there are few genetic differences between groups. Different groups may look very different, but, from a genetic perspective, they’re not very different at all:
85% of the total amount of genetic variation in humans occurs within a population (for example, Swedes)
8% occurs between populations within races (for example, in Caucasians, between Swedes and Italians)
7% occurs between races
If Earth were threatened with destruction and only one population—Swedes, for example—survived, 85% of the total amount of human genetic variation that exists today would still be present in that Swedish population.
Homo sapiens left Africa about 60kya. There has been little chance for genetic
differences to accumulate among geographically separated populations.
Humans have very little genetic variation; only about 1/1,000 base pairs among individuals.

25. Human Migratory Routes
Homo sapiens arose in Africa as determined by Cann’s mtDNA experiments. About 60,000 years ago, populations started to venture out through the Horn of Africa into the Middle East. The first phase of colonization took our ancestors through Asia and into Australia about 50,000 years ago. Not until about 15,000 years ago did the first modern humans cross from Siberia to North America to populate the New World.
Despite its closeness to the African mainland, the first humans arrived in Madagascar only about 2000 years ago, and the colonists came from Southeast Asia, not Africa.
Hawaii was colonized about 1500 years ago and New Zealand only 1000 years ago.
Because the spread of modern humans was so recent, there are few genetic differences between groups. Different groups may look very different, but, from a genetic perspective, they’re not very different at all:
85% of the total amount of genetic variation in humans occurs within a population (for example, Swedes)
8% occurs between populations within races (for example, in Caucasians, between Swedes and Italians)
7% occurs between races
If Earth were threatened with destruction and only one population—Swedes, for example—survived, 85% of the total amount of human genetic variation that exists today would still be present in that Swedish population.
85% of total genetic variation in humans occurs within a population (ie. Swedes)
8% between populations within races (I.e. in Caucasians between Swedes and Italians)
7% occurs between races 25

26. Natural Selection Explains Human Differences
Heat-adapted Masai in Kenya
Cold-adapted Inuit in Greenland
It is likely that natural selection is responsible for the physical differences between populations. For example, people with dark skin tend to live in lower latitudes with high levels of solar radiation, and people with light skin tend to live in higher latitudes with low levels of solar radiation. A likely factor in this observation is vitamin D, which is required for the production of bone. A deficiency of vitamin D can result in the skeletal malformation known as rickets. The body can synthesize vitamin D, but the process requires UV radiation. Heavily pigmented skin limits the entry of UV radiation into the cells, and so limits the production of vitamin D. This does not present a problem in parts of the world where there is plenty of sunlight, but it can be problematic in regions of low sunlight. Natural selection favored lighter skin in the ancestors of Eurasian populations because lighter skin favored the production of the vitamin.
In hot climates where dissipating body heat is a priority, a tall and slender body form has evolved. The body type of the East African Masai maximizes the ratio of surface area to volume and thus aids heat loss.
In colder climates, selection has favored a more robust, stockier body form, as seen in the Inuit, who have a low ratio of surface area to volume that promotes the retention of heat.
Tall, slender body
Dark skin, less entry of UV to produce Vitamin D, but lots of sunlight
Short, squat body
Light skin, more entry of UV to make Vitamin D in area of low sunlight

27. Influence of Culture on Evolution
24.5 Culture is a potent force for change in modern humans.
Culture has played a role in the process of evolution by allowing us to transcend our biological limits. One example of this is our ability to live in inhospitable parts of the world because of clothing and ingeniously constructed shelters.
Another example is our ability to tolerate lactose in our diets beyond the first few years of life after the domestication of cattle. In some populations, there has been subsequent human biological evolution in favor of lactose tolerance.
Inventions, such as clothing, have allowed our species to expand its geographic range.

28. Cultural Influence in Other Species
Cultural influence is not unique to humans.
For example:
1. Small birds called Blue Tits learned to peck through the aluminum caps of milk bottles left on doorsteps to reach the rich cream at the top of the milk, presumably by watching another bird do it.
2. Meerkats teach their young to handle prey.
3. Chimpanzees have learned to use tools to hunt insects.
Blue Tits learned to peck thru aluminum caps by watching other birds
Meerkats teach their young to handle prey
Chimps have learned to use tools to hunt insects.

29. Language
Chimps and bonobos are able to learn/use sign language to express words and simple sentences.
Language is not uniquely human. There are different languages for different species: The dance of worker bees and warning vocalizations of Vervet monkeys are examples.
Attempts have been made to explore the capabilities of chimpanzees by teaching them sign language or other forms of visual communication. Chimpanzees were able to acquire extensive vocabularies and construct simple sentences, but the process is difficult to evaluate because the animals’ achievements are a product of the training program.
While other animals possess simple versions of culture, language, and even consciousness, the capabilities of our species in all three areas are truly exceptional.